While Double Fine Productions already has "much of the technology in place to produce PC versions of all these games," studio founder Tim Schafer explains in the recently-updated DF FAQ (via Eurogamer), publishers simply "don't see enough financial reward" in bringing Double fun to PC gamers.
As a developer we do not have final say in the sku plan for our games. That is the decision of the person investing the money, i.e. the publisher. We have much of the technology in place to produce PC versions of all these games, but there is still some more work required to make them shippable and that costs money. So far, our publishers have not elected to fund that work. Not because they hate PC Gamers, but because they don't see enough financial reward. Double Fine does care about PC Gamers, and we always push for a PC version, and will continue to do so in the future. If we ever get super stinking rich here, with enough money to fund PC versions of our games, then we will go back and make them ourselves! Oh man, wouldn't that be cool?
Brutal Legend was ultimately published by Electronic Arts, having started off with Sierra Entertainment, while THQ is responsible for Costume Quest and Stacking.
And if you continue nagging Tim about PC releases? "Every time that happens it make my eye twitch and I take a dollar out of the 'PC Port Fund' jar."